Document the majormail 2026-06-07 incident: when userdb home == maildir
root, the LDA/Sieve duplicate database (.dovecot.lda-dupes + .locks) lands
inside the mail store and the maildir lister exposes it as phantom
mailboxes ("dovecot.lda-dupes"), logging stat(.../tmp) "Not a directory".
Fix: point home at a non-dotted subdir. Wired into the troubleshooting
index and SUMMARY.
